Method for making a tympanic membrane prosthesis

ABSTRACT

Synthetic anatomical members, such as a tympanic membrane and a malleus, are illustrated and a method for making them is disclosed. In accordance with the method, prepared collagenous tissue is placed in a mold and immersed in, or sprayed with, a buffered formaldehyde or functionally similar preservative. After removal from the mold, the tissue retains the shape assumed in the mold. Provisions are made for attaching other anatomical members, such as bone, to the tissue for use in, for example, en bloc reconstruction of an eardrum and ossicles.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to the shaping and forming ofbiological tissue for providing an implant for restoration of thestructure and/or function of an anatomical member and, in particular, toa synthetic tympanic membrane, an associated malleus or ossicularsubstitute and method of making the same.

Over the past 20 years many methods and tissues have been used to repairtympanic membrane perforations. These efforts have generally utilized ascaffolding of autogenous connective tissue from various sources. Thesurface of epithelial covering has been derived from a free or pedicledskin graft or from epithelium migration from the host's externalauditory canal skin. Dried (and, hence, nonviable) temporalis fascia incombination with epithelial migration from adjacent canal skin has beenthe most successful of this type of repair. Closure of the perforationis virtually assured with this technique but lateral displacement of thegraft and blunting of the anterior sulcus has resulted in thick tympanicmembranes in a small but significant percentage of cases. Thisundesirable effect frequently results in unsatisfactory hearingimprovement when it occurs.

When, in addition, restoration of one or more of the ossicles isrequired, in particular, the malleus, some additional surgicaldifficulty is encountered. This is due to the lack of any provision inthe conventional use of fascia tissue for preattaching a malleus orfunctionally similar member or for attachment of a host malleus to thetissue during surgery.

In addition, fascia tissue grafts do not ordinarily assume the conicalshape of a natural tympanic membrane. This is undesirable in that thenormal conical shape of the natural tympanic membrane is believed to beadvantageous for better hearing.

More recently, the closure of tympanic membrane perforations and thereconstruction of ossicles with fresh sterile homograft tympanicmembranes and ossicles has been attempted with varying degrees ofsuccess. In each case, temporal bone cores are taken at the time of anautopsy and dissected. The preparation of homograft tympanic membraneswith or without the malleus and other ossicles involves delicate,time-consuming and expensive procedures which are wholly dependent onthe availability of suitable donors.

The ready availability of an inexpensive synthetic tympanic membranewith or without one or more of the ossicles or functionally similarmember and having the physical shape and characteristics of a homografttympanic membrane is, therefore, highly desirable as is the availabilityof synthetic members for reconstructing other parts of the auditorysystem.

SUMMARY OF THE INVENTION

A principal object of the present invention is an apparatus and methodfor making synthetic anatomical members.

In accordance with this object, a principal feature of the presentinvention is a mold corresponding in size and shape to a desiredanatomical member. A section of prepared collagenous tissue is placed onor in the mold and is treated as by applying a fixative thereto forcausing the tissue to retain the shape of the mold.

In making a synthetic tympanic membrane, the preparation of thecollagenous tissue involves, in the case of brain covering tissue,separating the thicker dura mater from the thinner arachnoid dura. Oneor more straps of dura mater are left attached to the arachnoid dura forforming one or more slings for receiving a host or synthetic malleus. Asection of the arachnoid dura including the straps is then placed on themold. The assemblage is then sprayed with or immersed in bufferedformaldehyde or other functionally similar preservative.

Another feature of the present invention is the use of collagenoustissue for forming tissue molds within which living autogenous bonefragments are placed during surgery for reconstruction of bonyanatomical members.

The availability of collagenous tissue far exceeds the availability ofhomograft tympanic membranes and is in general much easier to obtain.For example, a supply of tissue from a single autopsy providessufficient tissue for making as many as twenty synthetic tympanicmembranes. Furthermore, it is believed, collagenous tissue from animalsources other than humans may also be used. This would avoid entirelythe problems associated with obtaining tissue from human donors.

These and other objects, features and avantages of the present inventionwill be apparent from the following detailed description andaccompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the human ear.

FIG. 2 is a diagrammatic representation of a section of collagenoustissue.

FIGS. 3 and 9 are diagrammatic representations of a section ofcollagenous tissue prepared in accordance with the present invention.

FIGS. 4 and 5 are perspective views of molds in accordance with thepresent invention.

FIG. 6 is a cross-sectional view taken in the direction of lines 6--6 ofFIG. 4.

FIG. 7 is a perspective view of a synthetic tympanic membrane with anatural malleus in situ made in accordance with the present invention.

FIG. 8 is a perspective view of a synthetic tympanic membrane and asynthetic malleus made in accordance with the present invention.

FIG. 10 is a perspective view of a synthetic tympanic membrane having apouch for receiving a malleus in accordance with the present invention.

DETAILED DESCRIPTION

Referring to FIG. 1, the ear can be divided into three portions: anouter ear, a middle ear, and an inner ear.

The outer ear is made up of an auricle 1 -- that part external to thehead -- and an external ear canal 2.

The middle ear is separated from the outer ear by a thinconically-shaped tympanic membrane (the eardrum) 3. The middle ear iscomposed of the tympanic membrane 3 and the ossicles, Three little earbones -- the malleus (hammer) 4, the incus (anvil) 5, and the stapes(stirrup) 6. The air space surrounding these little bones is known asthe middle ear space. This space is normally filled with air whichpasses through the Eustachian tube 7 from the throat 8 to the middleear. Immediately behind the middle ear space is a bony honeycomb of airpockets called the mastoid.

The third portion, the inner ear, is composed of two segments: a cochlea(the hearing canal) 10 and a labyrinth (balance canals) 11. Both arenormally filled with a watery solution which bathes the delicate nerveendings of a hearing and balance nerve, 12 and 13, respectively.

The structure and/or function of various ones of the above describedparts of the ear, such as the tympanic membrane, ossicles and posteriorear canal wall may be restored individually or en bloc, if necessary,using synthetic anatomical members made in accordance with the presentinvention. The synthetic members, comprising collagenous tissue, ashereinafter described, serve as a scaffolding over which natural tissuegrowth occurs. Also the characteristics of the members and the method offabricating them serve to provide the maximum possible restoration offunction.

Referring to FIG. 2, in the fabrication of a synthetic tympanic membranethere is provided a section of collagenous tissue 20. Tissue 20 istypically brain covering tissue comprising a layer of dura mater 21 on alayer of arachnoid dura 22. However, other tissue from other parts ofthe human anatomy, as well as, it is believed, from other animals, mayalso be used. This is because tissue rejection does not appear to be aproblem with homograft tympanic membranes; and synthetic membranes, aswith the homografts, serve as a scaffolding over which natural tissuegrowth occurs within a relatively short period of time.

Referring to FIG. 3, in the initial preparation of tissue 20, the duramater layer 21 is separated from the arachnoid dura layer 22 except fora pair of straps 23 and 24. A medial portion of each of straps 23 and 24is thereafter elevated off the arachnoid dura for forming a receivingspace 25 between the straps and the arachnoid dura layer. Alternatively,as shown in FIG. 9, a medial portion of a remnant layer of dura mater 21may be elevated off the arachnoid dura layer 22 to form a pouch 37. Thespace 25 and pouch 37 are provided for receiving a bone member ormalleus as hereinafter described with respect to FIGS. 7 and 8. As afurther alternative, slits may be provided in dura layer 22 forreceiving the bone member or malleus in lieu of both straps 23 and 24and pouch 37. After the initial preparation, the tissue is formed with amold into a shape corresponding to a natural tympanic membrane.

Referring to FIG. 4, there is provided a mold 30 comprising acylindrically shaped shaft 31, whch approximates the size and shape of ahuman external ear canal. One end of shaft 31 is provided with a surface33 corresponding to the shape of a natural tympanic membrane and asecond end of shaft 31 is provided with a base 34.

After being prepared as described with respect to FIGS. 3 and 9, asection of arachnoid dura including straps 23 and 24 or pouch 37 isplaced over the surface 33 of mold 30. An annular retaining sleeve 32 isthen fitted over the tissue for holding the tissue on the mold. The base34 serves as a means for holding the mold when placing the sleeve 32.Sleeve 32, however, may take other forms, such as a rubber O-ring or afemale sleeve corresponding in size and shape to shaft 31.Alternatively, the tissue may be held in place on the mold simply by thefingers of the operator preparatory to fixing for shaping said tissuethereon.

The assemblage of the mold 30 and tissue 22 is then air dried, ifdesired, and immersed in a container of or sprayed with an agent, suchas buffered formaldehyde or other functionally similar fixative such asgluteraldehyde, which causes the tissue to retain the configuration ofthe mold. Air drying is optionally used as it results in a thinning ofthe tissue, which may be desirable in some cases.

A suitable solution for fixing the tissue is 4% formaldehyde buffered toa pH of 5-7.0, although the percentage and pH are not critical to theinvention. The tissue remains exposed to the fixative solution untiladequate fixation has occurred rendering the new shape to be that of themold. The tissue is then removed from the mold and stored. The storagesolution has typically been 1/2% buffered formaldehyde at pH 7.0 butother solutions that maintain sterility could be used. When any bonetissue is incorporated as part of the desired end product, it isnecessary to use a solution that will not cause decalcification. Forthis reason, the above solution of pH 7.0 has been used.

In FIGS. 6-8, there is illustrated a synthetic tympanic membrane 40 madein accordance with the above procedure. Membrane 40 has a generallyconically shaped first wall 41 corresponding in size and shape to anatural tympanic membrane and a generally cylindrically shaped secondwall forming a skirt 35 with a shape similar to that of the medialportion of the external ear canal. The skirt 35, which may be trimmed tosuit a particular patient, serves to facilitate surgical implantation byproviding a ready means for interfacing with host tissue.

A natural malleus, indicated generally as 26 in FIG. 7, comprises a head27 and a shank 28. If restoration of a malleus is not required by apatient, the straps 23 and 24 of the synthetic tympanic membrane 40 areslipped over the shank 28 of the malleus of the patient and the skirt 35implanted in a conventional manner. In this regard, the adjacent earcanal tissue is elevated and placed over the skirt 35 to facilitatetissue migration onto the membrane. Alternatively, the patient's malleusmay be inserted in the pouch 37 of FIG. 10. As a further alternative, itshould be appreciated that frequently it is not necessary to use thestraps 23 and 24 or the pouch 37 for receiving a host malleus as it hasbeen found that a host malleus will often successfully adhere duringhealing to a properly placed synthetic membrane.

Referring to FIG. 8, a bone fragment 29, or a natural homograft malleusis provided in straps 23 and 24 in place of malleus 26 when areconstruction of a host malleus is required. Bone 29 is generallycylindrical in shape and may be provided with a means, such as anaperture (not shown), in which a prosthesis may be fitted for attachmentto an incus and/or stapes.

In addition to the use of a synthetic member in the restoration of atympanic membrane, the restoration of other anatomical members is alsonow possible. For example, restoration of an anterior wall portion ofthe mastoid (posterior wall of ear canal) or other bony members isaccomplished using an appropriately shaped mold of collagenous tissuewithin which is placed a viable autogenous filler of powdered orfragmented bone.

Referring to FIG. 5, there is provided, for example, a cylindricallyshaped mold 50 comprising a first cylindrically shaped member or shell51, a second shell 52 and a retaining cylinder 53. A larger section ofcollagenous tissue 20 is prepared as described with respect to FIG. 3,but without straps 23 and 24. A first section of tissue is laid on theconcave surface of shell 51. Shell 52 is then placed over the tissue. Asecond section of the tissue is folded about one end of shell 52 andlaid on the concave surface of shell 52. The retaining cylinder 53 isthen placed on the second section of tissue and the whole assemblagesecured as by a rubber O-ring. The tissue is then treated as describedabove with respect to the making of a synthetic tympanic membrane.

The tissue assumes and is found to retain the shape of the mold and maythereafter be surgically implanted. At the time of implantation, thetwo-ply piece is filled with viable autogenous bone from the recipient.The bony matrix thus formed is found to form in time a solid wall offirm bone which integrates with the adjacent host bone. Because of theperformed natural shape of the implant, the surgical procedure andpost-operative results are thereby enhanced.

Molds 30 and 50 are typically plastic, but may be made from a variety ofnon-reactive materials. It is also apparent that a variety of moldshapes may be used depending on the requirements of a particularsituation. For instance, the procedures described herein may be used inthe reconstruction of areas of missing dura, blood vessels and otherparts of the anatomy.

The invention has been described with respect to the use of braincovering tissue and a method of preparation involving splitting of thetissue. It is to be understood, however, that in some applications,unsplit brain covering tissue and other tissue types with or withoutsplitting may be used in practicing the present invention withoutdeparting from the scope thereof as hereinafter claimed.

What is claimed is:
 1. A method of making a biosynthetic tympanicmembrane and a biosynthetic tympanic membraneossicular substitute havinga concave and a convex surface, comprising the steps of:making a moldhaving the shape of a natural tympanic membrane; preparing collagenoustissue for placement on said mold; placing said tissue on said mold andshaping said tissue thereon; treating said tissue with bufferedformaldehyde, gluteraldehyde or another functionally similar fixativeagent on said mold for causing said tissue to retain the external shapeof said mold upon removal from said mold; forming a prosthetic receivingmeans of collagenous material on a surface of said tissue for receivingnatural and prosthetic ossicle members; and removing said treated tissuefrom said mold.
 2. A method according to claim 1 wherein said step offorming a prosthetic receiving means comprises forming a strap on saidconvex surface.
 3. A method according to claim 1 wherein said step offorming a prosthetic receiving means comprises forming a pouch on saidconvex surface.